shapelessFutureSequence.scala

import shapeless._
import scala.concurrent._
import scala.concurrent.ExecutionContext.Implicits.global

object FutureReducer extends Poly2 {
  implicit def f[A, B <: HList] = at[Future[A], Future[B]] { (f, resultFuture) =>
    resultFuture flatMap { r =>
      f map { v =>
        v :: r
      }
    }
  }
}

def hsequence[T <: HList](hlist: T)(implicit
  executor: ExecutionContext,
  folder: RightFolder[T, Future[HNil], FutureReducer.type]) = {
  hlist.foldRight(Future.successful[HNil](HNil))(FutureReducer)(folder)
}

val hFutures = Future.successful(5) :: Future.successful("string") :: HNil

hsequence(hFutures) // Works!

// =>
//   scala.concurrent.Future[shapeless.::[Int,shapeless.::[String,shapeless.HNil]]] = scala.concurrent.impl.Promise$DefaultPromise@1cc988d7


def tsequence[T <: Product, L <: HList, OL <: HList](tuple: T)(implicit
  executor: ExecutionContext,
  hlister: HListerAux[T, L],
  folder: RightFolderAux[L, Future[HNil], FutureReducer.type, Future[OL]]) = {
  hlister(tuple).foldRight(Future.successful[HNil](HNil))(FutureReducer)
}

val tFutures = (Future.successful(5), Future.successful("string"))

// This fails:
tsequence(tFutures)

// error: could not find implicit value for parameter folder: shapeless.RightFolderAux[L,scala.concurrent.Future[shapeless.HNil.type],FutureReducer.type,scala.concurrent.Future[OL]]
//   tsequence(tFutures)




// If I provide the type parameters explicitly, then it works
tsequence[
  (Future[Int], Future[String]),
  Future[Int] :: Future[String] :: HNil,
  Int :: String :: HNil](tFutures)

// =>
//   scala.concurrent.Future[shapeless.::[Int,shapeless.::[String,shapeless.HNil]]] = scala.concurrent.impl.Promise$DefaultPromise@53661dfd